Add rerun::external::ndarray (#7259)

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* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)

crates/store/re_types/src/lib.rs

@@ -262,6 +262,7 @@ pub mod external {
 
     pub use anyhow;
     pub use arrow2;
+    pub use ndarray;
     pub use uuid;
 
     #[cfg(feature = "ecolor")]